Magnetostrictive transducers are widely used for the liquid level measurement. See, for example, Koski et al., U.S. Pat. No. 4,839,590. Koski et al., discloses a measurement device for precision measurement of a liquid level in an underground storage tank that, in combination with a temperature measurement, allows detecting very small leaks from the tank. There is also a need for an accurate product density measurement in the same containers where the level is being measured.
A widely known method of density measurement is based on the hydrostatic principle that the weight loss of an object in a liquid equals the weight of the liquid displaced. The method is used in hydrometers where a relatively large weighted lower portion of a body is completely immersed into a liquid and a tall narrow upper section with a scale sticks out above the surface. The immersion depth of the hydrometer is in an inverse proportion to the liquid density. The hydrometer will float higher in a heavy liquid and lower in a light liquid. The sensitivity of the hydrometer is in an inverse proportion with a cross section of the upper section. The narrower this part is, the more sensitive the hydrometer is. The measurement range of the hydrometer is in direct proportion with the height of upper part. The taller that part is, the larger measurement range is.
There are also devices that combine level and density measurement in one magnetostrictive transducer. See, for example, Nyce et al., U.S. Pat. No. 5,253,522, and Russian patent RU 2138028.
The device disclosed in the Russian patent, and as generally illustrated in FIGS. 1a, 1b and 1c, includes a liquid density float 17, and a liquid level measurement float (not shown). The level measurement float is relatively less sensitive to liquid density variation and the liquid density float 17 is relatively more sensitive to liquid density variation. The liquid density float 17 is made in the form of an immersed cylinder 16 and four narrow vertical rods 15 that are located on top of the cylinder around its perimeter and extend above the surface. In essence, it is a group of four hydrometers connected together. The diameter of the density float 17 should be large enough to allow the liquid level float to freely move between the rods 15.
Size is one drawback of such a device. As discussed above, magnetostrictive transducers are widely used for leak detection in underground tanks. Such leak detection requires reliable measurement of very small changes of the liquid level, in the range of 0.001 inches (0.025 mm) or less. To achieve this type of resolution, the float for the level measurement should be heavy enough and therefore large enough to overcome the friction between the float and the body of the transducer, otherwise an effect known as “stiction” can mask a leak. At the same time, standard openings in the tanks for the transducer installation are typically four inches (100 mm) in diameter or less, which limits the permitted diameter of the float. To increase the tank opening size would be expensive.
To be able to combine level and density measurement into one transducer installed into a standard tank opening, without compromising leak detection capabilities, requires a density float to take as small portion of the opening diameter as possible and leave sufficient room for the level float.